Aromatic hydrocarbons can be processed to form product streams of a selected isomer of xylene. Xylene is an aromatic hydrocarbon that includes a benzene ring and two methyl substituents. Based on the structural position of the methyl substituents, three isomers of xylene can be formed: paraxylene, metaxylene, and orthoxylene. Paraxylene is a feedstock for terephthalic acid, which is used in the manufacture of synthetic fibers and resins. Metaxylene is used in the manufacture of certain plasticizers, azo dyes, and wood preservatives. Orthoxylene is a feedstock for phthalic anhydride, which is used in the manufacture of certain plasticizers, dyes, and pharmaceutical products.
For production of a desired xylene isomer, a mixed stream of the three xylene isomers is typically produced before the desired xylene isomer is separated. In other words, the desired xylene is not selectively produced but is selectively separated. A desired xylene isomer can be separated from mixed xylene streams by using an adsorbent selective to the desired isomer. After the desired isomer is adsorbed from the mixed xylene stream, the remaining isomers are discharged in a mixed raffinate stream. Typically, a desorbent desorbs the desired xylene isomer from the adsorbent, and the desorbent and selected xylene isomer are collected and separated by fractionation.
In the production of paraxylene, heavy desorbents are conventionally used to desorb the paraxylene from the adsorbent. Heavy desorbents are defined as having higher molecular weights and higher boiling points than xylene. Accordingly, light desorbents are defined as having lower molecular weights and lower boiling points than xylene. Heretofore, xylene isomer recovery systems using heavy desorbents have typically required less energy than systems with light desorbents, because the heavy desorbent does not require repeated evaporation and lifting during fractionation. However, heavy desorbent systems typically require stringent feed purity to control accumulation of undesired compounds in the recycled desorbent, such as impurities that reduce desorbent effectiveness and product purity. Further, additional equipment may be required to maintain heavy desorbent purity during the desorbent recycling process. Also, systems using heavy desorbent have fractionation columns with relatively higher reboiler temperatures. Higher reboiler temperatures lead to higher operating pressures that require higher pressure ratings for the equipment involved, thereby increasing the equipment capital cost.
Use of a light desorbent, such as the relatively inexpensive light desorbent toluene, relaxes feed specifications relative to systems using heavy desorbent. Cost savings for the relaxed feed specifications can offset the increased energy costs associated with recovering light desorbent as a fractionation column overhead. Xylene recovery apparatuses using light desorbent also provide savings in the total equipment count as desorbent purification and storage units are not necessary. Further, xylene recovery apparatuses using light desorbent have lower fractionation column operating pressures, allowing for less expensive thinner column shells with lower pressure ratings.
To efficiently produce a selected xylene isomer product from a hydrocarbon stream using light desorbent, it is generally desirable to separate substantially all of the aromatic hydrocarbons having eight carbon atoms (C8), including xylene and ethylbenzene, from the hydrocarbon stream and from recycled portions of the stream during processing to form a mixed xylene stream for separation into a selected xylene isomer stream. Conventional processes have attempted to eliminate aromatic C9 from the mixed xylene stream. This is particularly true of conventional heavy desorbent systems because the heavy desorbent becomes polluted by aromatic C9 during xylene isomer separation. Further, the energy needed to affect proper separation of xylene isomers is reduced when the aromatic C8 in the mixed xylene stream fed to the separation unit is made less pure. Despite this energy savings a relatively pure aromatic C8 feed stream for the separation unit is still desirable for the production of selected xylene isomer products.
Accordingly, it is desirable to provide methods and apparatuses for forming C8 aromatic streams with selected amounts of C9 aromatics. In addition, it is desirable to develop methods and apparatuses for efficiently producing selected xylene isomer products from hydrocarbon streams. Furthermore, other desirable features and characteristics of the present embodiment will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.